1. Technical Field
The present invention relates to a conveyance control device controlling conveyance of sheets.
2. Related Art
Japanese Patent Application Publication No. 09-208087 that is a patent literature discloses, as an image forming apparatus that forms an image on a sheet, a conveyance control device having a function of detecting a state in which a plurality of sheets is transferred in an overlapped state when sheets are transferred (so-called multiple feed).
In the conveyance control device of this type, a light-emitting element and a light-receiving element that are optical sensors are disposed to face each other across a transfer passage of sheets, and detect multiple feed based on change of a light-receiving amount by the light-receiving element.
Specifically, with reference to an amount of light passing through the sheet when only a normal one sheet is transferred, the amount of light when each sheet is transferred is compared to the reference. In the case of multiple feed, the amount of light passing through a sheet is smaller than when only a normal one sheet is transferred. Therefore, when the light-receiving amount by the light-receiving element is lowered, it is detected that the multiple feed has occurred.
Incidentally, the plurality of sheets to be transferred may include, in addition to a blank sheet, an image-formed sheet (so-called pre-printed sheet) on which a predetermined image has been previously formed and also image-formed sheets on which predetermined images different from each other are formed.
When the image-formed sheet is included in the sheets to be transferred, the light-receiving amount by the light-receiving element is lowered due to difference in display colors between sheets being transferred. As a result, there is such a problem that even if the multiple feed has not occurred, it may be erroneously detected.
A threshold value of the light-receiving amount for detecting the multiple feed is previously acquired for each image data corresponding to each sheet and, then, the threshold value is read out to detect the multiple feed while the each sheet is being transferred. However, switching the threshold value for each sheet may cause a problem of cumbersome processing.